C++11 thread_local implementation issue on Cygwin/AMD64
Václav Zeman
vhaisman@gmail.com
Mon Jan 19 15:28:00 GMT 2015
Hi.
I have hit an issue with thread-local storage variables on
Cygwin/AMD64, I do not see it with Cygwin/i686.
I am having linking issues when using `thread_local` keyword in Cygwin
with its GCC 4.8.3 and GCC 4.9.2. This is derived from log4cplus. The
test case is split into three files:
File def.hxx:
~~~~
#include <string>
namespace N
{
struct S { std::string str; };
// extern declaration in a header
extern thread_local S * ptd;
// accessing the extern declared ptd here
inline
S * get_ptd ()
{
if (! ptd)
ptd = new S;
return ptd;
}
} // namespace N
~~~~
File def.cxx:
~~~~
#include "def.hxx"
namespace N
{
// definition of ptd
thread_local S * ptd = nullptr;
} // namespace N
~~~
File use.cxx:
~~~~
#include "def.hxx"
namespace N
{
__declspec(dllexport)
void * foo ()
{
// invoking inline get_ptd() function to get the value in ptd
return get_ptd ();
}
}
~~~~
Now, when I compile each .cxx with `g++ -std=gnu++11
-fvisibility=hidden -c use.cxx def.cxx` and then try to link with `g++
-shared -o cygtest.dll use.o def.o`, I get the following error from
linker:
~~~~
use.o:use.cxx:(.text$_ZTWN1N3ptdE[_ZTWN1N3ptdE]+0x15): relocation
truncated to fit: R_X86_64_PC32 against undefined symbol `TLS init
function for N::ptd'
collect2: error: ld returned 1 exit status
~~~~
The nm -C ./def.o output confirms that:
~~~~
`--> nm -C ./def.o
0000000000000000 b .bss
0000000000000000 d .data
0000000000000000 r .rdata
0000000000000000 r .rdata$zzz
0000000000000000 t .text
0000000000000008 r __emutls_t._ZN1N3ptdE
0000000000000000 D __emutls_v._ZN1N3ptdE
0000000000000000 r std::piecewise_construct
~~~~
As you can see, the ptd thread-local variable initialization function
is not defined anywhere. The use.o references this initialization
function (see bottom of the listing):
~~~~
`--> nm -C ./use.o
0000000000000000 b .bss
0000000000000000 d .data
0000000000000000 i .drectve
0000000000000000 p .pdata
0000000000000000 p .pdata$_ZN1N1SC1Ev
0000000000000000 p .pdata$_ZN1N7get_ptdEv
0000000000000000 p .pdata$_ZTWN1N3ptdE
0000000000000000 r .rdata
0000000000000000 r .rdata$.refptr.__emutls_v._ZN1N3ptdE
0000000000000000 r .rdata$.refptr._ZTHN1N3ptdE
0000000000000000 r .rdata$zzz
0000000000000000 R .refptr.__emutls_v._ZN1N3ptdE
0000000000000000 R .refptr._ZTHN1N3ptdE
0000000000000000 t .text
0000000000000000 t .text$_ZN1N1SC1Ev
0000000000000000 t .text$_ZN1N7get_ptdEv
0000000000000000 t .text$_ZTWN1N3ptdE
0000000000000000 A .weak._ZTHN1N3ptdE._ZN1N1SC1Ev
0000000000000000 r .xdata
0000000000000000 r .xdata$_ZN1N1SC1Ev
0000000000000000 r .xdata$_ZN1N7get_ptdEv
0000000000000000 r .xdata$_ZTWN1N3ptdE
U __emutls_get_address
U __emutls_v._ZN1N3ptdE
U __gxx_personality_seh0
U __real__ZdlPv
U __real__Znwm
U _Unwind_Resume
U operator delete(void*)
0000000000000000 T N::S::S()
0000000000000000 T N::foo()
0000000000000000 T N::get_ptd()
U std::basic_string<char, std::char_traits<char>,
std::allocator<char> >::basic_string()
U operator new(unsigned long)
0000000000000000 r std::piecewise_construct
w TLS init function for N::ptd
0000000000000000 T TLS wrapper function for N::ptd
~~~~
Now, this code seems to work well on Linux with both GCC and Clang.
Is this a GCC problem on Cygwin?
Am I using extern thread_local wrong?
My experiments show that not using the extern keyword seems to fix the
issue. But I am not sure if that does not introduce two ptd
thread-local variables in two TUs.
See also http://stackoverflow.com/q/28023728/341065
--
VZ
-------------- next part --------------
#include "def.hxx"
namespace N
{
thread_local S * ptd = nullptr;
} // namespace N
-------------- next part --------------
#include <string>
namespace N
{
struct S { std::string str; };
extern thread_local S * ptd;
inline
S * get_ptd ()
{
if (! ptd)
ptd = new S;
return ptd;
}
} // namespace N
-------------- next part --------------
#include "def.hxx"
namespace N
{
__declspec(dllexport)
void * foo ()
{
return get_ptd ();
}
}
-------------- next part --------------
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